Reforming with hydrogen, or hydroforming, is a well established industrial process employed by the petroleum industry for upgrading virgin or cracked naphthas for the production of high octane products. Noble metal catalysts, notably platinum type catalysts to which additional metals have been added, are currently employed in reforming operations, reforming being defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics, isomerization of n-paraffins; isomerization of substituted aromatics; and hydrocracking of paraffins to produce gas and coke.
In a typical process, a series of reactors are provided with fixed beds of catalyst which receive downflow feed, and each reactor is provided with a heater because the reactions which take place are endothermic. A naphtha feed, with hydrogen recycle gas, is concurrently passed sequentially through a reheat furnace and then to the next reactor of the series. The vapor effluent from the last reactor of the series is a gas rich in hydrogen, which usually contains small amounts of normally gaseous hydrocarbons, and it is separated from the C.sub.5.sup.+ liquid product and recycled to the process to minimize coke production; coke invariably forming and depositing on the catalyst during the reaction.
Reforming catalysts are recognized as dual functional, the catalyst composite including a metal, or metals, or a compound or compounds thereof, providing a hydrogenation-dehydrogenation function and an acidic component providing an isomerization function. The platinum group or Group VIII noble metals (ruthenium osmium, rhodium, iridium, palladium, and platinum), particularly platinum, despite their expense have been recognized as having a combination of properties which make them particularly suitable for reforming operations, and platinum in particular has become widely used in commercial operations. Conventional reforming catalysts have thus long employed platinum composited with an inorganic oxide base, particularly alumina, and in recent years one or more promoters such as iridium, rhenium, germanium, tin, selenium, tellurium, etc., have been added to platinum to enhance one or more of certain of the characteristics which a good reforming catalyst must possess--viz., good activity, selectivity, activity maintenance, and yield stability. Halogen, e.g., chlorine, is generally added to enhance the acid function required of the catalyst, and the catalysts are sulfided prior to use.
Sulfided platinum-rhenium catalysts possess satisfactory activity for use in reforming operations, activity being defined as that property which imparts the ability to produce aromatics, aromatic production (or octane improvement) generally being measured as a function of temperature, feed rate, etc. Sulfided platinum-rhenium catalysts also possess satisfactory selectivity which is defined as that property which imparts the ability of the catalyst to produce high yields of high octane number C.sub.5.sup.+ liquid products with concurrent low production of normally gaseous hydrocarbons, i.e., C.sub.1 -C.sub.4 hydrocarbons, or solid carbonaceous by-products and coke. These catalysts also possess good stability or activity maintenance, i.e., activity plotted as a function of time, good stability or selectivity maintenance being defined as high retention of activity and selectivity, or continued high activity and stability for prolonged periods during reforming operations.
While any commercially viable reforming catalyst must possess all of these properties to a significant degree, no catalyst used in real world operations can possess all of these properties to the ultimate degree. One of these characteristics may be possessed by a catalyst in admirable degree, but the lesser quality of another of these characteristics may adversely affect the worth of the catalyst. Thus, a catalyst which possesses good selectivity does not necessarily have good activity, and vice versa. A small decrease in C.sub.5.sup.+ liquid yield can thus represent a large debit in commercial reforming operations. Conversely, the worth of a catalyst which possesses high selectivity may be jeopardized by the considerable capital cost which necessitates large charges of noble metals containing catalysts. Proper balance between these several properties is essential in the commercial world and an improvement gained in one property, or characteristic, cannot be too much offset by loss of another if the catalyst is to prove commercially viable.
In U.S. Pat. No. 4,149,991 and U.S. Pat. No. 4,169,785 there is disclosed catalyst composites which include platinum, rhenium, and tellurium for use in reforming. The addition of tellurium to platinum-rhenium reforming catalysts improves the activity of the catalysts. It also improves C.sub.5.sup.+ liquid yields by improving aromatics selectivity. Tellurium is analogous to sulfur in its action of suppressing hydrogenolysis when it is added to platinum-rhenium catalysts. A platinum-rhenium catalyst to which tellurium has been added, though it produces more methane and light gases than a sulfided platinum-rhenium catalyst, it produces greater yields of aromatics. For example, when a catalyst constituted of 0.3 wt. % Pt, 0.3 wt. % Re, and 0.06 wt. % Te which contained 1 wt. % chloride was placed in a reactor and used to reform a low sulfur (&lt;0.1 ppm S) paraffinic feed at 895.degree. F., 2.1 W/H/W, 150 psig, and 5000 SCF/B of hydrogen to produce a 100 RON product, the catalyst showed yield parity and substantial activity credits relative to a catalyst otherwise similar except that it was sulfided and contained no tellurium, run at similar conditions. Further, when the same platinum-rhenium-tellurium catalyst was employed in the last reactor of a multi-reactor unit at 950.degree.-978.degree. F. E.I.T. (equivalent isothermal temperature), 4.0 W/Hr/W, 140 psig, and 3600 SCF/B to produce a 100 RON product, a C.sub.5.sup.+ yield advantage was obtained at equivalent activity vis-a-vis a second, fully sulfided catalyst which contained 0.3 wt. % Pt and 0.3 wt. % Re at similar conditions. At these conditions, the latter sulfided catalyst would have been expected to provide better C.sub.5.sup.+ yield, but it did not due to the presence of the tellurium in the former catalyst. Hence, a platinum-rhenium-tellurium catalyst is known to provide at least equal or superior performance as contrasted with a sulfided platinum-rhenium catalyst containing the same weight distribution of platinum and rhenium metals.
Albeit the addition of tellurium has improved the performance of platinum-rhenium catalysts, further improvements of such catalysts are nonetheless desired; particularly catalysts of such character which will provide improved C.sub.5.sup.+ liquid yields.
Accordingly, it has now been discovered that a catalyst comprising catalytically active amounts of platinum, rhenium, and tellurium, composited with a porous inorganic base, notably alumina, is more selective, and more stable for producing high octane products from gasolines and naphtha at reforming conditions if it is pretreated in a sequence which includes the steps of oxidation and dry hydrogen reduction. The oxidation step is conducted by contacting the catalyst at an elevated temperature of at least about 850.degree. F., or temperature sufficient to form rhenium oxide, or rhenium oxide and other metal oxides dispersed upon the catalytic surface. The hydrogen reduction step is thereafter conducted by contact of the catalyst with dry hydrogen at conditions sufficient to remove product water from the catalyst as it is produced, and the reduction is continued until the stream of hydrogen gas leaving said catalyst (i.e., the exit gas) contains less than about 1000 parts per million of moisture (water) by volume, preferably less than about 500 ppm water, and more preferably less than 200 ppm water. In practicing the invention, the duration of contact of the catalyst with dry hydrogen is continued until the catalyst becomes dry, or desiccated; this state being reached when the hydrogen leaving said catalyst contains less than 1000 ppm water, preferably less than 500 wppm water, and more preferably less than 200 ppm water. This means, of course, that the dry hydrogen used for the reduction must contain less than 1000 ppm moisture, or less than 500 ppm moisture, and more preferably the hydrogen should be considerably drier, and should contain no more than 100 ppm water, preferably less than 50 ppm water.
The catalyst, in conducting the oxidation step, is suitably contacted with a flowing gas stream of an oxygen-containing gas, preferably air, suitably at temperatures ranging from about 850.degree. F. to about 1100.degree. F., preferably at temperatures ranging from about 950.degree. F. to about 1050.degree. F. Suitably, the period of treatment ranges from about 2 hours to about 10 hours, preferably from about 3 hours to about 4 hours. In the reduction step the oxidized metal surface of the catalyst is suitably contacted with a flowing stream of dry hydrogen, or a dry hydrogen-containing gas, at a temperature above about 900.degree. F., suitably at a temperature ranging from about 900.degree. to about 1050.degree. F., preferably from about 900.degree. to about 1000.degree. F. The time of reduction suitably ranges from about 0.5 hours to about 20 hours. Absolute pressures generally range from about atmospheric to about 400 pounds per square inch (psi), pressures ranging below about 100 psi being preferred.
The substantially complete reduction of the catalytic metal oxides to the zero valent state is believed to be essential, as disclosed in U.S. Pat. No. 4,369,129 which was issued Jan. 18, 1983 to Charles H. Mauldin and William C. Baird, Jr. This patent discloses a process for the pre-treatment of rhenium-containing catalysts, notably platinum-rhenium catalysts, in a sequence which necessarily includes the steps of oxidation, dry hydrogen reduction and sulfiding to obtain catalysts which provide higher performance, notably in increased C.sub.5.sup.+ liquid yields.
In accordance with the present invention it is only necessary to oxidize, and then reduce the platinum-rhenium-tellurium catalysts at temperatures greater than 850.degree. F. to obtain highly active, highly selective catalysts. Improved C.sub.5.sup.+ liquid yields can be obtained in reforming with platinum-rhenium-tellurium catalysts subjected to the aforesaid oxidation, and reduction treatment steps. Sulfiding of the catalyst prior to its use in reforming is unnecessary. Surprisingly, performance of platinum-rhenium-tellurium catalysts treated in accordance with this invention is improved to a greater degree than corresponding platinum-rhenium catalysts.